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Bernie Sanders derisively called Donald Trump "Mr. Macho" for backing out of a one-on-one debate

Sanders has hunkered down in California, the site of his last big match-up with Hillary Clinton

Washington (CNN)Bernie Sanders has a sarcastic new nickname for Donald Trump: "Mr. Macho."

The Vermont senator, who is hunkering down in California for his last showdown with Hillary Clinton in the Democratic presidential race, used the nickname as he bashed the presumptive Republican nominee for embracing -- and then backing away from -- the idea of a debate against Sanders.

"Let me not worry about Hillary Clinton right now. Let me worry about Donald Trump -- this big, brave, macho guy, my goodness," Sanders told supporters in Oakland on Monday. "He said he wanted to debate Bernie Sanders, then he said he didn't, and then he said he did, and then he said he didn't."

"So I say to Mr. Macho," he said, "that I am open to a debate anytime, any place here in the state of California, hopefully before the June 7 primary."

Trump's bestowing of nicknames on opponents drew waves of attention during the primary season and helped him emerge atop the Republican field. He routinely mentions "Crooked Hillary" about his likely general election opponent, Hillary Clinton. Trump's moniker for then-GOP rival Ted Cruz, "Lyin' Ted," helped define the Texas senator negatively.

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Sanders, though, is behind in the Democratic delegate count and unlikely to make it to a general election fight against the real estate mogul.

He has placed a big final bet on California -- one of five states to vote on June 7, the last major date on the primary calendar.

Clinton, who has a lead of 269 pledged delegates over Sanders -- that number grows by 494 when superdelegates are included -- is making a major push in California, as well, her campaign announced Monday. She'll campaign there from Thursday through Monday, spending five full days in the state.

Sanders said at the same event in Oakland that he has an "excellent chance" to win California, and "a chance, perhaps, to win big."

He also made the case that the campaign for the Democratic nomination doesn't end when California, New Jersey, New Mexico, Montana and South Dakota vote on June 7, underscoring his plan to take the race to the floor of the Democratic National Convention in Philadelphia in July.

Pointing to media expectations that the race ends after the June 7 contests, Sanders said: "That's not accurate. I don't think the DNC thinks that's accurate."

"She has received, obviously, a whole lot of superdelegate support, no question about that, a lot more than I have. But superdelegates don't vote until they're on the floor of the Democratic convention, that's when they vote," Sanders said.

"So our job, between -- you know, starting yesterday; our job is to convince superdelegates of one very, very important fact: That their most important responsibility is to make sure that the Democratic Party has the strongest candidate possible to defeat Donald Trump and make sure that man does not become president of the United States of America, which to my view would be a disaster."